Pwm signal generator, and inverter equipped with this pwm signal generator

ABSTRACT

The PWM signal generator of the present invention generates a first pulse waveform in which a first on-time ΔT 1  calculated by a first on-time calculator ( 401 ) is used as an on-duration, and a second pulse waveform in which a second on-time ΔT 2 , calculated by a second on-time calculator ( 402 ) when a preset delay time has elapsed from the start of the calculation of the first on-time ΔT 1 , is used as an on-duration. Also, a PWM signal generator ( 413 ) generates a PWM signal on the basis of a composite pulse in which the generated first pulse waveform and second pulse waveform are combined, and the first on-time calculator ( 401 ) calculates the first on-time ΔT 1  at the end of the composite pulse waveform.

TECHNICAL FIELD

This invention relates to a PWM signal generator that generates pulsewaveforms and to an inverter device equipped with this PWM signalgenerator.

BACKGROUND ART

Conventionally, system-linked inverter devices have been developed withwhich power was supplied by linking DC power generated by a fuel cell,solar cell, or the like to a commercial power system. With thissystem-linked inverter device, a technique is proposed for reducingswitching loss by reducing the number of times switching elements haveto switch.

For example, Japanese Patent Application Laid-open No. H11-53042discloses a technique for reducing switching loss by reducing the numberof times switching elements have to switch in the region of practicaluse. Such reduction is achieved by setting the frequency of a triangularwave for generating a PWM (pulse width modulation) signal that controlsthe on/off operation of the switching elements to 20 kHz when theinverter output power Pout is not within a range of 30 to 80% of a ratedvalue Pr, and changing the frequency of the triangular wave to a lowerfrequency (such as 15 kHz) when the inverter output power Pout is withina range of 30 to 80% of the rated value Pr.

Also, a method called current control by hysteresis is known as a methodwith which the number of times the switching elements have to beswitched can be reduced more than with the method disclosed in JapanesePatent Application Laid-Open No. H11-53042.

This current control by hysteresis method involves generating a PWMsignal by the method shown in FIG. 15, and controlling the on/offswitching of the switching elements with this PWM signal.

In FIG. 15, the solid line curve A indicates the waveform of a controltarget value of a fundamental wave component of output current, whilethe dotted line curves AU and AD indicate the upper and lower limitwaveforms within the allowable range when the fundamental wave componentof the output current fluctuates. Also, the zigzag line B indicated bythe one-dot chain line is the waveform of the current value outputtedfrom an inverter device.

With current control by hysteresis, when the current value outputtedfrom the inverter device rises to an upper limit value Iup of theallowable range ΔI, the level of the PWM signal is switched to the levelat which the switching elements are controlled so that the supply ofcurrent power to the inverter is stopped (labeled “low level” in FIG.15). On the contrary, when the current value outputted from the inverterdevice drops to a lower limit value Idown of the allowable range ΔI, thelevel of the PWM signal is switched to the level at which the switchingelements are controlled so that current power is supplied to theinverter is stopped (labeled “high level” in FIG. 15).

Patent Document 1: Japanese Patent Application Laid-open No. H11-53042

Guidelines are proposed for linking a system-linked inverter device to asystem. For instance, it is required to hold the effective value of thefundamental wave component (60 Hz in the Kansai area, and 50 Hz in theKanto area) within a specific allowable range with respect to the outputcurrent, or to keep each of the fifth, seventh, and thirteenth orderhigh-frequency components within 10 or less, and to keep them totallywithin 3% or less.

Examples of performance aspects generally required for an inverterdevice include higher output precision, faster response, and higherefficiency. Since the main object of a system-linked inverter device isto supply power to a system, unlike with an inverter device that is usedto control a motor, the need for higher efficiency is given priorityover higher output precision or faster response. Therefore, with asystem-linked inverter device, on the condition that the above-mentionedguidelines be satisfied, it is preferable to increase efficiency byreducing as much as possible the number of times switching.

Current control by hysteresis is a method in which the current valueoutputted from a system-linked inverter device is kept as much aspossible within an allowable range ΔI of the control target value (suchas a control target value of ±3%) to lower the switching frequency ofthe switching elements and reduce switching loss. Therefore, from thestandpoint of raising efficiency on the condition that guidelines bemet, it is considered that current control by hysteresis is a PWM signalgeneration method that is more suited to a system-linked inverter devicethan the method described in Japanese Patent Application Laid-open No.H11-53042.

However, current control by hysteresis involves the following problems.

(1) A circuit is necessary to constantly monitor whether the AC currentactually outputted from the system-linked inverter device departs fromthe allowable range or not.

(2) It is difficult to construct a unit in which a pattern of PWMsignals is generated according to whether the AC current actuallyoutputted from the system-linked inverter device departs from theallowable range, using a digital control system. Accordingly, gooduniversality and flexibility cannot be taken advantage of in the designof the digital control system.

DISCLOSURE OF THE INVENTION

The present invention has been proposed under the above situation, andit is an object of the present invention to provide a PWM signalgenerator that solves the drawbacks to current control by hysteresis andwhich generates PWM signals with a longer period by means of a digitizedcontrol system, and an inverter device that is equipped with this PWMsignal generator.

To solve the above-mentioned problems, the following technological meansare employed in the present invention.

The PWM signal generator provided by a first aspect of the presentinvention comprises a first pulse waveform generator for generating afirst pulse waveform; a second pulse waveform generator for generating asecond pulse waveform when a preset delay time elapses after a start ofgeneration of the first pulse waveform; and a PWM signal generator forgenerating a PWM signal based on a composite pulse waveform in which thefirst pulse waveform generated by the first pulse waveform generator iscombined with the second pulse waveform generated by the second pulsewaveform generator; wherein the first pulse waveform generator generatesa next first pulse waveform at an end of the composite pulse waveform.

With this constitution, since a commercial power system is generatedthat has a longer period than the first pulse waveform generated by thefirst pulse waveform generation means, a PWM signal with a longer periodcan be generated.

In a preferred embodiment of the present invention, the first pulsewaveform has a preset first pulse period, and becomes a high level in amiddle portion of the first pulse period and becomes a low level at bothends of the first pulse period. The second pulse waveform has a presetsecond pulse period, and becomes a high level in a first part of thesecond pulse period and becomes a low level in a latter part of thesecond pulse period. The composite pulse waveform is a same type ofwaveform as the first pulse waveform constructed by connecting thesecond pulse waveform to a high level duration of the first pulsewaveform.

In a preferred embodiment of the present invention, the first pulseperiod is equal to the second pulse period.

In a preferred embodiment of the present invention, the high levelduration of the first pulse waveform is disposed in a middle of thefirst pulse period.

In a preferred embodiment of the present invention, the delay timesatisfies a condition that the generation of the second pulse waveformstarts in a duration in which the first pulse waveform generated by thefirst pulse waveform generator is a high level.

In a preferred embodiment of the present invention, the delay time isone-half of the first pulse period.

In a preferred embodiment of the present invention, the first pulsewaveform generator includes: a first on-time calculator for computing,at a start of the first pulse period, a first on-time in which the firstpulse waveform is to be a high level; and a first inversion timingdecider for determining a first inversion timing at which a level of thefirst pulse waveform inverts from a low level to a high level in thefirst pulse period, based on the first on-time and a position of thehigh level in the first pulse period. The second pulse waveformgenerator includes: a second on-time calculator for computing, after aelapse of the delay time after a start of the first pulse period, asecond on-time in which the second pulse waveform is to be at a highlevel; and a second inversion timing decider for determining, based onthe second on-time, a second inversion timing at which a level of thesecond pulse waveform inverts from a high level to a low level in thesecond pulse period in which the second on-time has been computed. ThePWM signal generation means includes: an inversion timing detector fordetecting the first and second inversion timings with reference to astart timing of the first pulse period; and a PWM signal output unit forsetting an output level to the low level at the start of the first pulseperiod, subsequently inverting the output level to the high level whenthe first inversion timing is detected, inverting thereafter the outputlevel to the low level when the second inversion timing is detected,thereby generating a pulse signal in which the first pulse waveform andthe second pulse waveform are combined, for outputting this pulse signalas the various pulses of the PWM signal.

In a preferred embodiment of the present invention, the first inversiontiming decider determines, as the first inversion timing, a point when aremaining time, obtained by subtracting one-half of the computed firston-time from a time at the middle position of the high level in thefirst pulse period, has elapsed from a start of calculation of the firston-time, every time the first on-time is computed. The second inversiontiming determination means determines, as the second inversion timing, apoint when the computed second on-time has elapsed from a start ofcalculation of the second on-time, every time the second on-time iscomputed.

In a preferred embodiment of the present invention, the PWM signalgenerator further comprises: a determiner for determining whether thelevel of the second pulse waveform is a high level or not, every time aperiod of the first pulse waveform ends; and a pulse waveformregenerator for causing, only when the level of the second pulsewaveform at an end of a period of the first pulse waveform is a highlevel, the second pulse waveform generator to generate a second pulsewaveform again at an end of a period of the first pulse waveform. ThePWM signal generator generates a PWM signal based on a composite pulsewaveform in which the first pulse wave form is combined with thegenerated second pulse waveform and the regenerated second pulsewaveform.

In a preferred embodiment of the present invention, the PWM signalgenerator further comprises: a second determiner for determining whethera level of the second pulse waveform regenerated at an end of a periodof the previously generated second pulse waveform is a high level or notwhen the generation of the second pulse waveform is performed again bythe pulse waveform regenerator. The pulse waveform regenerator repeatsan operation of causing the second pulse waveform generator to generatea second pulse waveform again at an end of a period of the previouslygenerated second pulse waveform, until a level of the second pulsewaveform regenerated at an end of a period of the previously generatedsecond pulse waveform reaches a low level. The PWM signal generatorgenerates a PWM signal based on a composite pulse waveform in which thefirst pulse waveform is combined with the generated second pulsewaveform and one or more regenerated second pulse waveforms.

In a preferred embodiment of the present invention, the first pulsewaveform generator includes: a first-on time calculator for computing,at a start of the first pulse period, a first on-time in which the firstpulse waveform is to be at a high level; and a first inversion timingdecider for determining a first inversion timing at which a level of thefirst pulse waveform inverts from a low level to a high level in thefirst pulse period based on the first on-time and a position of a highlevel in the first pulse period. The second pulse waveform generatorincludes: a second on-time calculator for computing a second on-time inwhich the second pulse waveform is to be at a high level, after a elapseof the delay time from a start of the first pulse period, and if ageneration of the second pulse waveform is performed again by the pulsewaveform regenerator, at an end of the first pulse period and at an endof the period of the previously generated second pulse waveform; and asecond inversion timing decider for determining, based on the secondon-time that has been last computed by the second on-time calculator, asecond inversion timing at which a level of the second pulse waveforminverts from a high level to a low level in the second pulse period inwhich the second on-time has been computed. The PWM signal generatorincludes: an inversion timing detector for detecting the first andsecond inversion timings with reference to a start time of the firstpulse period; and a PWM signal output unit for setting an output levelto a low level at a start of the first pulse period, subsequentlyinverting the output level to the high level when the first inversiontiming is detected, holding the output level at the high level on thebasis of the one or more generated second pulse waveforms, subsequentlyinverting the output level to the low level when the second inversiontiming is detected, thereby generating a pulse signal in which the firstpulse waveform and the one or more second pulse waveform are combined,for outputting this pulse signal as the various pulses of the PWMsignal.

In a preferred embodiment of the present invention, the first inversiontiming decider determines, as the first inversion timing, a point whenthe remaining time, obtained by subtracting one-half of the computedfirst on-time from the time at the middle position of the high level inthe first pulse period, has elapsed from the start of calculation of thefirst on-time, every time the first on-time is computed. The secondinversion timing decider determines, as the second inversion timing, apoint when the second on-time last computed has elapsed from the startof the last calculation of the second on-time.

In a preferred embodiment of the present invention, the first on-timecalculator computes the first on-time by using a first calculationformula for finding a solution to a first state equation in which aninput variable is the first on-time of the first pulse waveform andwhich is derived from a state equation in which a state variableinputted to a control object is the first pulse waveform. The secondon-time calculator computes the second on-time by using a secondcalculation formula for finding a solution to a second state equation inwhich an input variable is the second on-time of the second pulsewaveform and which is derived from a state equation in which the statevariable inputted to the control object is the second pulse waveform.

The inverter device provided by a second aspect of the present inventioncomprises a DC power supply that outputs DC voltage; a bridge circuitwhich inversely converts the DC voltage outputted from the DC powersupply into AC voltage, and in which a plurality of switching elementsare bridge-connected; a control circuit that controls the inverseconversion operation of the bridge circuit by controlling an on/offoperation of the plurality of switching elements; a filter circuit thatremoves switching noise included in the AC voltage outputted from thebridge circuit; and a transformer that receives the AC voltage outputtedfrom the filter circuit for applying a transformed voltage to a load;wherein the control circuit includes the PWM signal generator accordingto the first aspect, and controls an on/off operation of the pluralityof switching elements by means of PWM signals generated by the PWMsignal generator.

In a preferred embodiment of the present invention, the DC power supplycomprises a solar cell, the bridge circuit comprises a three-phasebridge circuit, and the AC voltage outputted from the transformer isthree-phase AC voltage outputted in connection with a commercial powersystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the circuit configuration according to anembodiment of a single-phase inverter device which the present inventionrelates to;

FIG. 2 is a diagram illustrating the method for generating the PWMsignal that is generated by an inverter controller;

FIG. 3 is a diagram illustrating two kinds of pulse waveforms used ingenerating the PWM signal;

FIG. 4 is a diagram illustrating the pulse voltage that indicatesinverter output voltage;

FIG. 5 is a circuit diagram showing a model of a simplified inverterdevice;

FIG. 6 is a block diagram of the PWM signal generation function of theinverter controller;

FIG. 7 is a diagram illustrating the function of the switchingcomponent;

FIG. 8 is a flowchart of the procedure of generating a PWM signal in theinverter controller;

FIG. 9 is a diagram illustrating the output pulse waveform according toa second embodiment;

FIG. 10 is a diagram illustrating the output pulse waveform according toa third embodiment;

FIG. 11 is a diagram illustrating the output pulse waveform according toa fourth embodiment;

FIG. 12 is a diagram illustrating the circuit configuration according toan embodiment of the three-phase inverter device which the presentinvention relates to;

FIG. 13 is a block diagram of the basic configuration of the PWM signalgeneration component of a conventional three-phase inverter device;

FIG. 14 is a block diagram of the basic configuration of the PWM signalgeneration component of the three-phase inverter device according to thepresent invention; and

FIG. 15 is a diagram illustrating a method of current control byhysteresis.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described withreference to the drawings.

FIG. 1 is a diagram of the circuit configuration according to anembodiment of an inverter device which the present invention relates to.The inverter device 1 shown in the figure is a single-phasesystem-linked inverter device that supplies electric power based on DCpower in connection with a commercial power system.

The inverter device 1 includes a DC power supply 2 that outputs DCpower, an inverter circuit 3 that converts the DC power outputted fromthis DC power supply 2 into AC power, an inverter controller 4 thatcontrols the on/off operation of switching elements TR1 to TR4 in thisinverter circuit 3, a filter circuit 5 that removes switching noiseincluded in the AC voltage outputted from the inverter circuit 3, atransformer 6 for combining the AC voltage outputted from the filtercircuit 5 with system voltage for outputting the voltage to a system 9(corresponds to the load with respect to the inverter device 1), anoutput current detector 7 that detects current outputted from thetransformer 6 (hereinafter referred to as “output current”), and asystem voltage detector 8 that detects the voltage of the system 9(corresponds to the load with respect to the inverter device 1).

The inverter controller 4 controls the generated voltage from theinverter circuit 3, and thereby the inverter device controls the outputcurrent to conform with a target current for linking to the system. Theinverter controller 4 converts the output current and the target currentinto output voltage and target voltage, and uses a signal generated byprescribed calculation using these voltages to control the generatedvoltage from the inverter circuit 3. A characteristic of the presentinvention is this calculation using the output voltage and targetvoltage at the inverter controller 4, so description of the processingin which the output current and target current are converted into outputvoltage and target voltage will be omitted below for the sake ofsimplicity.

The DC power supply 2 is provided with a solar cell 211 that convertssolar energy into electrical energy. A diode D1 provided in an outputline of the solar cell 211 serves to prevent backflow of current fromthe inverter circuit 3 to the solar cell 211.

The inverter circuit 3 is constituted of a voltage control type ofinverter circuit. Specifically, the inverter circuit 3 includes fourswitching elements TR1 to TR4 in bridge connection. The switchingelements TR1, TR2, TR3, and TR4 are connected in parallel to feedbackdiodes D2, D3, D4, and D5, respectively. These switching elements canbe, for example, bipolar transistors, field effect transistors,thyristors, or other such semiconductor switching elements, and FIG. 1shows an example of using transistors.

DC voltage Vdc outputted from the DC power supply 2 is supplied to bothends of the serial connection of the switching element TR1 and theswitching element TR2 and the serial connection of the switching elementTR3 and the switching element TR4, and AC voltage converted by theinverter circuit 3 is outputted from the connection point a between theswitching element TR1 and the switching element TR2 and the connectionpoint b between the switching element TR3 and the switching element TR4.

The on/off operation of the four switching elements TR1 to TR4 iscontrolled by PWM signals outputted from the inverter controller 4.Specifically, two pairs of PWM signals with different pulse widths areoutputted from the inverter controller 4, where one pair is constitutedof two PWM signals with mutually inverted phases. When the PWM signalsof one pair are represented by S11 and S12, and the PWM signals of theother pair by S21 and S22, the PWM signals S11 and S12 are inputted tothe control terminals (the base of the transistor in FIG. 1) of theswitching element TR1 and the switching element TR2, respectively, whilethe PWM signals S21 and S22 are inputted to the control terminals (thebase of the transistor in FIG. 1) of the switching element TR3 and theswitching element TR4, respectively.

When the on state of the switching elements TR1 to TR4 is taken to be a“conductive state,” and the off state is taken to be a “shutoff state,”the serial connection of the switching element TR1 and the switchingelement TR2 of the inverter circuit 3 (hereinafter this circuit portionwill be referred to as a “first arm”) is such that the operating stateis alternately repeated between a state of (TR1, TR2)=(on, off) and astate of (TR1, TR2)=(off, on). As is clear from the bridge connection inFIG. 1, the state of (TR1, TR2)=(on, off) is a circuit state in which DCpower is supplied from the solar cell 211 to the inverter circuit 3,while the state of (TR1, TR2)=(on, off) is a circuit state in which thesupply of DC power to the inverter circuit 3 is shut off.

Similarly, the serial connection of the switching element TR3 and theswitching element TR4 (hereinafter this circuit portion will be referredto as a “second arm”) is such that the operating state is alternatelyrepeated between a state of (TR3, TR4)=(on, off) and a state of (TR3,TR4)=(off, on). The state of (TR3, TR4)=(on, off) is a circuit state inwhich DC power is supplied from the solar cell 211 to the invertercircuit 3, while the state of (TR3, TR4)=(off, on) is a circuit state inwhich the supply of DC power to the inverter circuit 3 is shut off.

The periods of the PWM signals S11 and S12 and the PWM signals S21 andS22 change, but these periods change in synchronization with each other,and only the duty ratios are mutually different. For example, at a givenperiod, if the duty ratio of the PWM signals S11 and S12 is greater thanthe duty ratio of the PWM signals S21 and S22 (that is, if theon-duration of the PWM signal S11 is longer than the on-duration of thePWM signal S21), then the circuit state of (TR1, TR2)=(on, off) islonger than the circuit state of (TR3, TR4)=(on, off), so the voltage Vaat the connection point a between the switching element TR1 and theswitching element TR2 is higher than the voltage Vb at the connectionpoint b between the switching element TR3 and the switching element TR4.In this case, with the connection point b being taken to be the voltagereference point (0 V), for example, then a voltage of (Va−Vb) (>0) isoutputted from the inverter circuit 3.

Conversely, if the duty ratio of the PWM signals S11 and S12 is lessthan the duty ratio of the PWM signals S21 and S22, then the circuitstate of (TR1, TR2)=(on, off) is shorter than the circuit state of (TR3,TR4)=(on, off), so the voltage Va at the connection point a is lowerthan the voltage Vb at the connection point b, and a voltage of (Va−Vb)(<0) is outputted from the inverter circuit 3.

Since the duty ratio of the PWM signals S11 and S12 and the duty ratioof the PWM signals S21 and S22 change continuously for every period, thevoltage Vout that is outputted from the inverter circuit 3 and passesthrough the filter circuit 5 ends up varying in a sinusoidal shape.

The inverter controller 4, as discussed above, includes two PWM signalgenerators 41 and 42 corresponding to the first and second arms, andthese PWM signal generators 41 and 42 generate the four PWM signals S11,S12, S21, and S22. The DC/AC conversion operation of the invertercircuit 3 is controlled by the PWM signals S11, S12, S21, and S22. Theinverter controller 4 is mainly constituted of a microprocessor. Theinverter controller 4 uses data about the output voltage converted fromthe output current inputted from the output current detector 7 toexecute prescribed calculation with a preset program, and therebycalculates the on and off timing of the PWM signals S11 and S21,switches between the high and low levels in real time on the basis ofthis calculation result, and thereby generates the PWM signals S11 andS21. The inverter controller 4 also generates the PWM signals S12 and822 by inverting the phase of these PWM signals S11 and S21. The methodfor generating the PWM signal S11 or the PWM signal S21 will bediscussed below.

The filter circuit 5 is constituted of a low-pass filter in which twoinductors L_(F1) and L_(F2) are respectively connected in series to apair of output lines, and a capacitor C_(F) is connected in parallel tothe output side. In FIG. 1, the filter circuit 5 is represented as anbalanced circuit, so the same inductors L_(F1) and L_(F2) arerespectively connected in series to the pair of output lines, but ifrepresented by an unbalanced circuit, then the inductors L_(F) (that is,L_(F1)+L_(F2)) and the capacitor C_(F) are connected in an inverted Lshape.

The AC voltage outputted from the inverter circuit 3 contains switchingnoise of the switching elements TR1 to TR4 produced by the PWM signals,and to eliminate this switching noise, the cutoff frequency of thefilter circuit 5 should be set to the lowest frequency of the PWMsignals or further lower. However, as will be discussed below, since theperiod of the PWM signals pertaining to this embodiment is extendedaccording to the circumstances, the lowest frequency cannot bespecified. Therefore, a favorable frequency that is lower than thelowest frequency of the range that can be expected from experience (2kHz, for example) and is higher than the frequency of the system voltage(50 or 60 Hz) is set as the cutoff frequency of the filter circuit 5.

The transformer 6 raises or lowers the AC voltage outputted from thefilter circuit 5 (sinusoidal voltage) to substantially the same level asthe system voltage. The output current detector 7 is provided on one ofthe pair of output lines of the transformer 6, and detects the ACcurrent (output current) flowing to that output line. The system voltagedetector 8 is provided between the two ends of the pair of output linesof the transformer 6, and detects the AC voltage (output voltage)outputted from those output lines. The output voltage of the inverterdevice 1 is controlled to so as to be substantially the same as thevoltage of the system 9, so the voltage detected by the system voltagedetector 8 can also be called the voltage of the system 9. The outputcurrent detected by the output current detector 7 is inputted to theinverter controller 4 and converted into output voltage, and is utilizedto generate the PWM signals S11 and S21. The output voltage detected bythe system voltage detector 8 is also inputted to the invertercontroller 4, and is utilized to detect the phase.

Next, the method for generating PWM signals in the inverter controller 4will be described.

The inverter device 1 is a system that includes the inverter circuit 3,which performs nonlinear operation, the filter circuit 5, and thetransformer 6, which perform linear operation (that is, it is a mixedsystem of linear and nonlinear circuits). According to modern controltheory, it is proposed that a digital control system be constructed by amethod in which the control system is modeled as a linear system, astate equation that expresses this linear system model is produced, anda control value is found as the solution to this state equation.

With the present invention, attention is focused on the fact that thevoltage inputted to the first linear circuit of the inverter device 1 ispulse voltage, and a PWM holding method is applied by presetting theperiod and waveform of the pulse voltage. Specifically, a time T_(ON) atwhich the pulse voltage reaches the high level in a preset samplingduration T (hereinafter referred to as the “on-time”) is termed a statevariable, the control system of the inverter device 1 is expressed by astate equation based on the position of the high level duration of thison-time T_(ON) (hereinafter referred to as the “on-duration”), and thisstate equation is converted into a state equation for a discrete timesystem and solved to find the on-time T_(ON). This state equation andthe method for solving it will be discussed below. A control system forthe inverter device 1 (a control system that generates PWM signals) canbe constructed as a digital control system by employing a method inwhich pulses of PWM signals are generated from the calculated on-timeT_(ON), the sampling duration T, and the position of the on-durationwithin the period.

With the above method, however, the sampling period T must be selectedas the period T of the pulse voltage, and since this period T is fixed,the period of each PWM signal pulse cannot be varied. This is because ifthe on-time T_(ON) is calculated at the start of the sampling period T,the state of the inverter system at the time of this calculation isassumed not to change within the sampling period T, and it is a premisethat the sampling period T will be controlled with a pulse voltagehaving the on-duration of the calculated on-time T_(ON).

In the present invention, the on-time T_(ON) is calculated at the startof the sampling duration T, and the waveform of the pulse voltage isdetermined until the start of the next sampling duration T, but theon-time T_(ON) is recalculated within the sampling duration T, and thetiming at which the on-duration of the pulse voltage is ended (the offtiming) is corrected. If the state has not changed within the samplingduration T, the off timing obtained from the recalculated on-time T_(ON)(the corrected off timing) should be substantially the same as the offtiming obtained from the initially calculated on-time T_(ON) (theinitial off timing), but if the state has changed within the samplingduration T, then the corrected off timing will differ from the initialoff timing. Regardless of whether or to the corrected off timing changeswith respect to the initial off timing, the previously determined pulsewaveform is corrected by the pulse waveform determined by therecalculated on-time T_(ON). Consequently, the period of each PWM signalpulse is longer than the sampling period T.

Pulse waveforms include the following.

(1) A waveform that is at the low level on both sides of one period, andat the high level in the middle portion (a type that is inverted to thehigh level midway through the period, and then returns to the low level;hereinafter referred to as “A type”).

(2) A waveform that is at the high level in the first part of oneperiod, and at the low level in the latter part (a type that is invertedto the high level at the start of the period, and is inverted to the lowlevel in the middle of the period; hereinafter referred to as “B type”).

(3) A waveform that is at the low level in the first part of one period,and at the high level in the latter part (a type that is inverted to thelow level at the start of the period, and is inverted to the high levelin the middle of the period; hereinafter referred to as “C type”).

These types A, B, and C are the possible combinations of pulse waveformpreviously determined and pulse waveforms determined by recalculation.However, if, for example, the previously determined pulse waveform andthe pulse waveform determined by recalculation are both A type, thecorrected pulse waveform will have two pulses in a period, and theperiod of the PWM signal pulses will be shorter than the sampling periodT. Therefore, to lengthen the period of the PWM signal pulses, it isnecessary to use type B, in which recalculation performed in theon-duration of the previously determined pulse waveform, and the pulsewaveform determined by recalculation is at the high level at the startof the period.

Also, for example, if the previously determined pulse waveform and thepulse waveform determined by recalculation are both B type, theon-duration of the previously determined pulse waveform will be suchthat the end time of the on-duration is uncertain in the first part ofthe period, so the timing at which recalculation is performed is limitedto the duration only in the first part of the period of the previouslydetermined pulse waveform. In this case, the previously performedcalculation and the recalculation are spaced closer together, and theperiod of the corrected pulse waveform cannot be lengthened very much.Also, if the previously determined pulse waveform is C type and thepulse waveform determined by recalculation is B type, the on-duration ofthe previously determined pulse waveform will be such that the starttime of the on-duration is uncertain in the latter part of the period,so the timing at which recalculation is performed is limited to theduration only in the latter part of the period of the previouslydetermined pulse waveform. Because of the above, the optimal combinationis one in which the previously determined pulse waveform is A type andthe pulse waveform determined by recalculation is B type.

The periods of the previously determined pulse waveform and the pulsewaveform determined by recalculation may be different from one another,but it is preferable for them to be the same in order to simplify thecalculation processing. Also, there are no limitations on the positionof the on-duration of the previously determined pulse waveform, but itis preferably disposed in the middle of the period in order to improvecalculation processing accuracy. Nor are there limitations on the timingof recalculation, as long as it is within the on-duration of thepreviously determined pulse waveform, but it is preferably a timing inthe middle of the period of the previously determined pulse waveform inorder for the calculation processing for the previous calculation andthe recalculation to be constant.

The method for generating the PWM signals generated by the invertercontroller 4 will now be described in detail with reference to FIGS. 2and 3. The following is a description of a first embodiment in which thepreviously determined pulse waveform is A type, the pulse waveformdetermined by recalculation is B type, the periods of the pulsewaveforms are the same, the position of the on-duration of thepreviously determined pulse waveform is disposed in the middle of theperiod, and the timing of the recalculation is in the middle of theperiod of the previously determined pulse waveform.

FIG. 2 is a diagram illustrating the method for generating the PWMsignal that is generated by the inverter controller 4. FIG. 2( a) is awaveform diagram showing how two kinds of pulse waveform are combinedinto an output pulse waveform. FIG. 2( b) is a diagram showing therelation between the output pulse waveform and the output current I ofthe inverter circuit 3, and corresponds to FIG. 15.

FIG. 3 is a diagram illustrating two kinds of pulse waveforms used ingenerating the PWM signal. The following description will be of the PWMsignal S11, but the same method can be applied to the generation of thePWM signal S21 as well.

First, in FIG. 2, the on-time is calculated for when a preset initialperiod T (such as 0.17 ms) is used as one period, at a time instant oft=t₀. This calculation is intended to find the on-time of the pulsewaveform of a PWM signal for holding the output current I within theallowable range, and the on-time is calculated from the on-timecalculated before, the output voltage converted from the output currentdetected by the output current detector 7, and the target voltage. Thecalculation formula for this on-time is found from the state equation ofthe inverter device 1, and the method for calculating this calculationformula will be discussed below.

Next, a pulse waveform is generated in which the on-duration of thecalculated on-time is located in the middle thereof (see FIG. 3( a);hereinafter referred to as the “first pulse waveform”; also, the on-timecomputed to generate the first pulse waveform is referred to as the“first on-time”). If the calculated first on-time is represented by ΔT₁,this first pulse waveform comes on after the elapse of the time(1/2)·(T−ΔT₁) since the time instant t₀ at which this ΔT₁ wascalculated, and goes off after the elapse of the time (1/2)·(T+ΔT₁) (seethe pulse waveform on the left side of the pattern 1 in FIG. 2).

Next, at t=t₁ after the elapse of the time (1/2)·T from the time instantt₀, the on-time is again computed for when T is assumed to be oneperiod.

In the example in FIG. 2( b), I₁ is the output current I at t=t₁, andthe optimal on-time is calculated so that the output current I at thispoint will fall within the allowable range ΔI. In the case where thetime for calculating on-time is too short and the on-time is notcalculated, since the previously generated first pulse waveform cannotbe extended, the first pulse waveform generated as mentioned above isused as the output pulse waveform without the following processing beingperformed.

If the on-time is calculated at t=t₁, a pulse waveform is generated inwhich the on-duration of the calculated on-time is located at theleading edge (see FIG. 3( b); hereinafter referred to as the “secondpulse waveform”; also, the on-time calculated to generate the secondpulse waveform is referred to as the “second on-time”). With thecalculated second on-time being represented by ΔT₂, this second pulsewaveform becomes on from the time instant t₁ at which the on-time wascalculated, and goes off after the elapse of ΔT₂ (see the pulse waveformon the left side of the pattern 2 in FIG. 2).

The inverter controller 4 generates the above-mentioned output pulsewaveform in which two pulse waveforms are combined, and PWM signals areoutputted on the basis of the output pulse waveforms (see the pulsewaveform on the left side of the output pulse waveform in FIG. 2). Thisoutput pulse waveform is one in which the on-time is (1/2)·ΔT₁+ΔT₂,which becomes on after the elapse of the time (1/2)·(T−ΔT₁) since thetime instant t=t₀ (hereinafter this time instant will be referred to as“T_(a)”), and becomes off after the elapse of the time (1/2)·(T+ΔT₂)(hereinafter this time instant will be referred to as “T_(b)”). As isclear from FIG. 2, the period of this output pulse waveform is (3/2)·T.

At t=t₀, the level of the PWM signal outputted from the invertercontroller 4 is controlled on the basis of the output pulse waveformgenerated in the previous calculation processing, and becomes the lowlevel in FIG. 2. After t=t₀, basically the level of the PWM signaloutputted from the inverter controller 4 is controlled on the basis ofthe first pulse waveform calculated in the calculation processing at tt₀, so the PWM signals outputted from the inverter controller 4 is heldat the low level, and the level of the PWM signal is inverted from thelow level to the high level at t=t_(a) on the basis of the calculationresult at t=t₀.

The level of the PWM signal inverted to the high level continues untilt=t_(a)+ΔT₁, so the level of the PWM signal at t=t₁ is held at the highlevel. When ΔT₂ is calculated in calculation processing at t=t₁, thesecond pulse waveform based on this ΔT₂ is at the high level from t=t₁to t=t_(b) (=t₁+ΔT₂), and at the low level from t=t_(b) to t t₃, so thelevel of the PWM signal is held at the high level even beyond t=t₁, andis inverted to the low level based on the second pulse waveform att=t_(b).

The effect of controlling the level of the PWM signal outputted from theinverter controller 4 is that the output current I of the invertercircuit 3 is as shown by N, which is the solid line in FIG. 2( b). Ifcalculation processing is not performed for correcting the on-time att=t₁, and calculation processing of the next pulse waveform is insteadperformed at t=t₂, the output current I of the inverter circuit 3 is asshown by N′, which is indicated by a broken line in FIG. 2( b).

As discussed above, the first pulse waveform is at the high level in themiddle of the period T, and at the low level at both ends, socalculation processing for finding the first pulse waveform must beperformed at the timing at which the PWM signal is at the low level. Onthe other hand, as discussed above, the second pulse waveform is at thehigh level at the leading edge of the period T, and is at the low levelat the trailing edge, so in the present invention, basically a firstpulse waveform is generated, after which calculation processing isperformed for generating a second pulse waveform during the time thatthe PWM signal is at the high level on the basis of this first pulsewaveform, and if a second pulse waveform is obtained, the period of theoutput pulse waveform is extended by combining this second pulsewaveform with the first pulse waveform.

Thus, when the second on-time ΔT₂ is calculated by recalculation, theperiod of the output pulse waveform is extended by (1/2)ΔT from theinitial period T. This allows the inverter controller 4 to generate PWMsignals in which the period of the pulses is extended. Therefore, withthe inverter device 1 pertaining to this embodiment, the number of timesthe switching elements TR1 to TR4 have to be switched is reduced andthere is less switching loss than when the period of the pulses of thePWM signals is fixed at T, so the voltage conversion efficiency can beimproved.

Also, in this embodiment, the calculation timing of the various on-timesis fixed, and measurement values inputted from the outside are used onlyduring calculation of the on-times, so the measurement values do notneed to be monitored constantly. Also, since the inverter controller 4is constituted by a digital control system, this affords greateruniversality and flexibility in design.

Also, in this embodiment, the on-duration of the first pulse waveform isdisposed in the middle of the period. Therefore, it is less likely thatthe start time for the on-duration of the first pulse waveform haspassed when the first on-time ΔT₁ is calculated, or that thison-duration extends beyond one period, or that other such problems isencountered. And since the error is smallest in the calculation formulafor the first on-time ΔT₁, this improves the accuracy of the calculatedfirst on-time ΔT₁.

Also, in this embodiment, the calculation of the second on-time ΔT₂ isperformed in the middle of the period of the first pulse waveform.Therefore, it is less likely that the on-duration of the first pulsewaveform has ended at the start time for the on-duration of the secondpulse waveform, or that other such problems is encountered. Also, sincethe calculation periods for the first on-time ΔT₁ and the second on-timeΔT₂ are constant, control accuracy is better.

Also, in this embodiment, a method is employed for further extending theperiod of the output pulse waveform.

If the second on-time ΔT₂ is shorter than (1/2)·T, then the PWM signalis at the low level at the end point of the period T used for computingthe first pulse waveform. Also, the output pulse waveform at which thePWM signal is at the low level is found from the second pulse waveformfor the duration of (1/2)·T extended from the end point of the period T.Therefore, the calculation processing for the next first pulse waveformis performed at the point when this extended duration (1/2)·T has ended(see the waveforms of patterns 1 and 2 for t=t₀ to t₃ in FIG. 2( a)).

On the other hand, if the second on-time ΔT₂ of the second pulsewaveform is longer than (1/2)·T, since the PWM signal will be at thehigh level at the end point of the period T used for computing the firstpulse waveform, calculation processing for the second pulse waveform canbe performed again at the end point of this period T. In view of this,in this embodiment, if the second pulse waveform is at the high level atthe end point of the period T of the first pulse waveform, calculationprocessing for the second pulse waveform is performed again, and if theon-time is calculated in this calculation processing, the second pulsewaveform from the second time will be further combined with the pulsewaveform combined with the second pulse waveform of the first time, andthis further extends the period the output pulse waveform.

Specifically, at t=t₃ in FIG. 2, calculation processing is performed forthe next first pulse waveform, and after a first pulse waveform having afirst on-time ΔT₁′ is generated, calculation processing is performed forthe second pulse waveform at t=t₄ (=t₃+(1/2)·T). The second on-time ΔT₂′calculated in this calculation processing is greater than (1/2)·T, so att=t₅ a second on-time ΔT₃′ is again computed. If ΔT₃′ is not calculatedhere, a pulse waveform in which the first pulse waveform of the on-timeΔT₁′ (see the pulse waveform on the right side of the pattern 1 in FIG.2) and the second pulse waveform of the on-time ΔT₂′ (see the pulsewaveform on the right side of the pattern 2 in FIG. 2) are combined isgenerated as the output pulse waveform.

If ΔT₃′ is calculated at t=t₅, and if ΔT₃′ is shorter than (1/2)·T, thena pulse waveform in which the first pulse waveform of the on-time ΔT₁′,the second pulse waveform of the on-time ΔT₂′, and the second pulsewaveform the on-time ΔT₃′ are combined is generated as the output pulsewaveform (see the pulse waveform on the right side of the output pulsewaveform in FIG. 2). This output pulse waveform comes on after theelapse of the time (1/2)·(T−ΔT₁′) since the time instant t=t₃(hereinafter this time instant will be referred to as “t_(c)”), goes offafter the elapse of the time T ΔT₃′ (hereinafter this time instant willreferred to as “t_(d)”), and is a pulse waveform in which the on-time is(1/2)·T+(1/2)·ΔT₁′+ΔT₃′. Also, as is clear from FIG. 2, the period ofthis output pulse waveform is 2·T.

The output current I of the inverter circuit 3 is as shown by N, whichis the solid line in FIG. 2( b). If calculation processing is notperformed for correcting the on-time at t=t₄, the output current I ofthe inverter circuit 3 is as shown by N″, which is indicated by a brokenline in FIG. 2( b). Also, if calculation processing is not performed forcorrecting the on-time at t=t₅, the output current I of the invertercircuit 3 is as shown by N′″, which is indicated by a broken line inFIG. 2( b).

If ΔT₃′ is greater than (1/2)·T, the PWM signal is at the high level atthe end point of the period T used in computing the second pulsewaveform, so the second on-time is again computed for the end point t=t₆(=t₅+(1/2)·T) of this period T. Thereafter, the second on-time iscalculated in the same manner, and if this on-time is greater than(1/2)·T, calculation of the second on-time is performed again at thetime instant reached after the elapse of (1/2)·T from the time instantat which this calculation processing was performed. This calculationprocessing of the second on-time is repeated until either the secondon-time is not calculated or the calculated second on-time is less thanor equal to (1/2)·T.

Thus, in this embodiment, the on-time and period of the output pulsewaveform are extended as long as the second on-time is calculated andthis second on-time is greater than (1/2)·T, that is, as long as thesecond pulse waveform is at the high level after the end point of theperiod T of the previous second pulse waveform. Consequently, the lengthof the period of each pulse of the PWM signal is (1/2)·mT (where m is anatural number greater than or equal to 2), and compared to when theperiod of the PWM signal is fixed at T, the switching elements TR1 toTR4 are switched fewer times on average, switching loss is reduced, andthe voltage conversion efficiency can be improved.

The calculation of on-time will now be described.

In this embodiment, in order to generate the PWM signal mainly bycalculation processing, a PWM holding method is used to model thecircuits of the inverter circuit 3 to the transformer 6 in the inverterdevice 1 as a linear system as described above. Specifically, in thisembodiment the state equation in which the output voltage (pulsevoltage) of the inverter circuit is used as input is modified, a stateequation in which the on-time of the pulse of said output voltage isused as input (a state equation representing a linear system model) isderived, a formula for obtaining a solution is found from this stateequation, and this formula is used to compute the on-time substantiallyin real time.

First, the method for deriving a state equation in which the on-time ofthe output pulse is used as input from a state equation in which theinverter output voltage is used as input will be described.

In modern control theory, various methods have been studied foranalyzing the operating characteristics of a control object by finding anumerical model of a control object and an input/output relation forthat numerical model, deriving an equation for the operating state(state equation), and solving this state equation.

In the case of a one-input, one-output system in which the controlobject is expressed by the following differential state equations (1)and (2), it is known that the solutions for the state variable x(t) andthe output y(t) are expressed by the following formulas (3) and (4).

[E1]

{dot over (x)}(t)=A·x(t)+B·u(t)  (1)

y(t)=C·x(t)+D·u(t)  (2)

u(t): input vector

y(t): output vector

x(t): state variable vector

{dot over (x)}(t): derivative of x(t)

A, B, C, D: coefficient vectors

[E2]

x(t)=e ^(A(t−t) ⁰ ⁾ ·x(t ₀)+∫_(t) ₀ ^(t) e ^(A(t−τ)) ·B·u(τ)dτ  (3)

y(t)=C·e ^(A(t−t) ⁰ ⁾ ·x(t ₀)+∫_(t) ₀ ^(t) C·e ^(A(t−τ))·B·u(τ)dτ+D·u(t)  (4)

x(t₀): initial value of state variable

The input vector u(t) with the inverter device is the pulse voltagev_(i)(t) outputted from the inverter circuit. The period of this inputpulse is represented by T, and the state in which t=(k+1)·T will beconsidered from the state of t₀=kT. In formula (3) above, if u(t) istaken to be v_(i)(t), and t₀=kT and t=(k+1)·T, the following formula (5)is obtained.

[E3]

x((k+1)T)=e ^(AT) x(kT)+∫_(kT) ^((k+1)T) e ^(A((k+)1)T−τ)·B·v_(i)(τ)dτ  (5)

Here, as shown in FIG. 4, the inverter output voltage that is the inputamount is pulse voltage with a size of V_(DC) and a width of ΔT, and hasan A type pulse waveform in which the on-time is disposed in the middle.Therefore, if kT≦τ<kT+(1/2)·(T−ΔT) and kT+(1/2)·(T+ΔT)≦τ<(k+1)·T, thenv_(i)(τ)=0, and if kT+(1/2)·(T−ΔT)≦τ<kT+(1/2)·(T ΔT), thenv_(i)(τ)=V_(DC). Consequently, formula (5) above is modified into thefollowing formula (6). Thus, the input parameter can be converted fromthe voltage of the inverter into the pulse width.

$\begin{matrix}\left\lbrack {E\; 4} \right\rbrack & \; \\{{x\left( {\left( {k + 1} \right)T} \right)} = {{^{AT}{x({kT})}} + {\int_{\frac{({T - {\Delta \; T}})}{2}}^{\frac{({T + {\Delta \; T}})}{2}}{{^{A{({T - \tau})}} \cdot B \cdot V_{DC}}\ {\tau}}}}} & (6)\end{matrix}$

When formula (6) above is modified into the following formula (7),supposing x[k]=x(kT), the following formula (8) is obtained. Thus, theinverter device 1 is expressed as a linear system in which the input isthe on-time ΔT.

$\begin{matrix}\left\lbrack {E\; 5} \right\rbrack & \; \\\begin{matrix}{{x\left( {\left( {k + 1} \right)T} \right)} = {{^{AT}{x({kT})}} + {A^{- 1} \cdot \left\{ {^{\frac{A \cdot {({T + {\Delta \; T}})}}{2}} - ^{\frac{A \cdot {({T - {\Delta \; T}})}}{2}}} \right\} \cdot B \cdot V_{DC}}}} \\{= {{^{AT}{x({kT})}} + {A^{- 1} \cdot ^{\frac{A \cdot T}{2}} \cdot \left\{ {^{\frac{{A \cdot \Delta}\; T}{2}} - ^{\frac{{{- A} \cdot \Delta}\; T}{2}}} \right\} \cdot B \cdot V_{DC}}}} \\{= {{^{AT}{x({kT})}} + {^{\frac{A \cdot T}{2}} \cdot {\int_{\frac{{- \Delta}\; T}{2}}^{\frac{\Delta \; T}{2}}{^{{A \cdot \Delta}\; T}\ {\Delta}\; {T \cdot B \cdot V_{DC}}}}}}} \\{= {{^{AT}{x({kT})}} + {^{\frac{A \cdot T}{2}} \cdot {\int_{kT}^{{({k + 1})}T}{\Delta \; {T(k)}\ {\Delta}\; {T \cdot B \cdot V_{DC}}}}}}}\end{matrix} & (7) \\\left\lbrack {E\; 6} \right\rbrack & \; \\{{{x\left\lbrack {k + 1} \right\rbrack} = {{A_{T}{x\lbrack k\rbrack}} + {B_{T}\Delta \; {T\lbrack k\rbrack}}}}{A_{T} = ^{AT}}{B_{T} - {^{\frac{A \cdot T}{2}} \cdot B \cdot V_{DC}}}} & (8)\end{matrix}$

Next, a formula for computing the on-time from a state equation for aspecific inverter device will be found. The electrical circuit formulaof a model of the simplified inverter device shown in FIG. 5 isexpressed by the following formula (9) from Kirchhoff's law. v_(i)(t)and v_(o)(t) are voltage values at time t for points V_(i) and V₀ inFIG. 5, respectively.

$\begin{matrix}\left\lbrack {E\; 7} \right\rbrack & \; \\{{\frac{\;}{t}\begin{bmatrix}{V_{0}(t)} \\{{\overset{.}{V}}_{0}(t)}\end{bmatrix}} = {{\begin{bmatrix}0 & 1 \\{- \frac{1}{LC}} & {- \frac{1}{RC}}\end{bmatrix}\begin{bmatrix}{V_{0}(t)} \\{{\overset{.}{V}}_{0}(t)}\end{bmatrix}} + {\begin{bmatrix}0 \\\frac{1}{LC}\end{bmatrix}{{Vi}(t)}}}} & (9)\end{matrix}$

By applying the above-mentioned PWM holding method to formula (9) above,and taking the input to be the on-time ΔT[k], the following formula (10)is obtained. To simplify the calculation of the various elements in thematrix, they are expressed by φ₁₁, φ₁₂, φ₂₁, φ₂₂, g₁, and g₂.

$\begin{matrix}\left\lbrack {E\; 8} \right\rbrack & \; \\{\begin{bmatrix}{V_{0}\left\lbrack {k + 1} \right\rbrack} \\{{\overset{.}{V}}_{0}\left\lbrack {k + 1} \right\rbrack}\end{bmatrix} = {{\begin{bmatrix}\varphi_{11} & \varphi_{12} \\\varphi_{21} & \varphi_{22}\end{bmatrix}\begin{bmatrix}{V_{0}\lbrack k\rbrack} \\{{\overset{.}{V}}_{0}\lbrack k\rbrack}\end{bmatrix}} + {\begin{bmatrix}g_{1} \\g_{2}\end{bmatrix}\Delta \; {T\lbrack k\rbrack}}}} & (10)\end{matrix}$

In this embodiment, control is performed by deadbeat control. In thecase of deadbeat control, the formula for computing the on-time ΔT[k]can be found by expanding formula (10) above. By expanding formula (10)above, the following formulas (11) and (12) are obtained. Multiplyingboth sides of the following formulas (11) and (12) by φ₂₂ and φ₁₂,respectively, and substituting (k+1) for k, the following formulas (13)and (14) are obtained.

[E9]

V ₀ [k+1]=φ₁₁ V ₀ [k]+φ ₁₂ {dot over (V)} ₀ [k]+g ₁ ΔT[k]  (11)

{dot over (V)} ₀ [k+1]=φ₂₁ V ₀ [k]+φ ₂₂ {dot over (V)} ₀ [k]+g ₂ΔT[k]  (12)

φ₂₂ V ₀ [k]=φ ₁₁φ₂₂ V ₀ [k−1]+φ₂φ₂₂ {dot over (V)} ₀ [k−1]+g ₁φ₂₂ΔT[k−1]  (13)

φ₁₂ {dot over (V)} ₀ [k]=φ ₁₂φ₂₁ V ₀ [k−1]+φ₁₂φ₂₂ {dot over (V)} ₀[k−1]+g ₂φ₁₂ ΔT[k−1]  (14)

By combining the above formulas (13) and (14), the following formula(15) is obtained. By substituting this for the above formula (11) andmodifying, the following formula (16) is obtained.

$\begin{matrix}\left\lbrack {E\; 10} \right\rbrack & \; \\{{\varphi_{12}{{\overset{.}{V}}_{0}\lbrack k\rbrack}} = \begin{matrix}{{\left( {{\varphi_{12}\varphi_{21}} - {\varphi_{11}\varphi_{22}}} \right){V_{0}\left\lbrack {k - 1} \right\rbrack}} +} \\{{\left( {{g_{2}\varphi_{12}} - {g_{1}\varphi_{22}}} \right)\Delta \; {T\left\lbrack {k - 1} \right\rbrack}} + {\varphi_{22}{V_{0}\lbrack k\rbrack}}}\end{matrix}} & (15) \\{{\Delta \; {T\lbrack k\rbrack}} = \frac{\begin{Bmatrix}{{V_{0}\left\lbrack {k + 1} \right\rbrack} - {\left( {\varphi_{11} + \varphi_{22}} \right){V_{0}\lbrack k\rbrack}} +} \\{{\left( {{\varphi_{11}\varphi_{22}} - {\varphi_{12}\varphi_{21}}} \right){V_{0}\left\lbrack {k - 1} \right\rbrack}} +} \\{\left( {{g_{1}\varphi_{22}} - {g_{2}\varphi_{12}}} \right)\Delta \; {T\left\lbrack {k - 1} \right\rbrack}}\end{Bmatrix}}{g_{1}}} & (16)\end{matrix}$

Using the above formula (16), the on-time ΔT[k] during the presentsampling can be calculated from the on-time ΔT[k−1] and the outputvoltage v₀[k−1] during the previous sampling, the output voltage v₀[k]during the present sampling, and the target output voltage v₀[k+1]during the next sampling.

The calculation formula for the on-time ΔT[k] described above is forcalculating the on-time ΔT₁[k] of the first pulse waveform in which theon-duration in FIG. 3( a) is located in the middle. The calculationformula for the second on-time ΔT₂[k] of the second pulse waveformhaving a B type pulse waveform in which the on-duration in FIG. 3( b) islocated at the leading edge side can be found in the same way, using aformula in which B_(T)=e^(AT)·B·V_(DC) in formula (8).

The above-mentioned calculation formula is one in a case of usingdeadbeat control. The present invention can also be applied to othertypes of control, but when another type of control is used, the on-timeΔT[k] must be calculated by a method that is compatible with thatcontrol.

In the above description, it was described that an output pulse waveformcomprising a combination of two pulse waveforms was generated, in orderto explain the concept of the method for generating PWM signals.Actually, the inverter controller 4 is constituted by the functionalblocks shown in FIG. 6, the on timing is set from the calculated firston-time ΔT₁[k], the off timing is set from the second on-time ΔT₂[k+r](where r is the number of times the second on-time has beenrecalculated) that was last calculated, and the output level of the PWMsignal is switched at these timings.

FIG. 6 is a block diagram of the PWM signal generation function of theinverter controller 4.

The inverter controller 4 includes, as function blocks for generatingPWM signals, a first on-time calculator 401, a second on-time calculator402, a memory component 403, a target voltage setting component 404, aswitching adjuster 405, a switching component 406, a comparator 407, acounter 408, an initial period setting component 409, an on timingsetting component 410, an off timing setting component 411, a timemeasurer 412, and a pulse signal generator 413.

The first on-time calculator 401 calculates the first on-time ΔT₁[k].The first on-time calculator 401 calculates the first on-time ΔT₁ [k]when a time measurement signal has been inputted from the time measurer412 in the case where a select signal has been inputted from theswitching component 406. The first on-time calculator 401 uses the aboveformula (16), which is a calculation formula for the first on-timeΔT₁[k], to calculate the first on-time ΔT₁[k] at the current samplingfrom the output voltage v₀[k] obtained by A/D conversion from the outputvoltage signal inputted and converted from the output voltage detector7, the output voltage v₀[k−1] used in the previous calculation andinputted from the memory component 403 (hereinafter referred to as the“previous output voltage”), the on-time ΔT[k−1] calculated the previoustime (hereinafter referred to as the “previous on-time”), and the targetvoltage v₀[k+1] inputted from the target voltage setting component 404.

The first on-time calculator 401 outputs the first on-time ΔT₁[k]calculated by calculation to the on timing setting component 410 and theoff timing setting component 411, and outputs a switch signal to theswitching component 406 and a reset signal to the counter 408. The firston-time calculator 401 also outputs the calculated first on-time ΔT₁[k]and the output voltage v₀[k] used in calculation to the memory component403. The first on-time ΔT₁[k] and the output voltage v₀[k] here areutilized as the previous on-times ΔT₁[k] and ΔT₂[k] and the previousoutput voltage v₀[k] in calculating the first on-time ΔT₁[k+1] or secondon-time ΔT₂[k+1] at the next sampling.

The second on-time calculator 402 calculates the second on-time ΔT₂[k].The second on-time calculator 402 calculates the second on-time ΔT₂[k]when a time measurement signal has been inputted from the time measurer412 in the case where a select signal has been inputted from theswitching component 406. The second on-time calculator 402 uses acalculation formula for the second on-time ΔT₂[k], which is the same asthe above formula (16), to calculate the second on-time ΔT₂[k] from theoutput voltage v₀[k] obtained by A/D conversion from the output voltagesignal, the output voltage v₀[k−1] used in the previous calculation andinputted from the memory component 403, the on-time ΔT[k−1] calculatedthe previous time, and the target voltage v₀[k+1] inputted from thetarget voltage setting component 404.

The second on-time calculator 402 outputs a count signal to the counter408 when the second on-time ΔT₂[k] is calculated, and outputs thecalculated second on-time ΔT₂[k] to the comparator 407 and the offtiming setting component 411. The second on-time calculator 402 alsooutputs to the memory component 403 the calculated second on-time ΔT₂[k]and the output voltage v₀[k] used for calculation. The second on-timeΔT₂[k] and the output voltage v₀[k] here are utilized as the previouson-times ΔT₁[k] and ΔT₂[k] and the previous output voltage v₀[k] incalculating the first on-time ΔT₁[k+1] or second on-time ΔT₂[k+1] at thenext sampling.

The memory component 403 stores the output voltage v₀[k] and theon-times ΔT₁[k] and ΔT₂[k] inputted from the first on-time calculator401 or the second on-time calculator 402 by overwriting them over thestored output voltage (the previous output voltage v₀[k−1]) and theon-times (the previous on-times ΔT₁[k−1] and ΔT₂[k−1]). The memorycomponent 403 also outputs the stored output voltage v₀[k] and on-timesΔT₁[k] and ΔT₂[k] to the first on-time calculator 401 and the secondon-time calculator 402 as the previous output voltage and previouson-times.

The target voltage setting component 404 outputs to the first on-timecalculator 401 and the second on-time calculator 402 the target voltagecorresponding to the target waveform of the preset output voltage, whena time measurement signal is inputted from the time measurer 412.

The switching adjuster 405 adjusts the switching of the switchingcomponent 406 in order to adjust the time it takes from the calculationof the on-time ΔT₂ of the second pulse waveform by the second on-timecalculator 402 until the calculation of the on-time ΔT₁ of the firstpulse waveform by the first on-time calculator 401. The switchingadjuster 405 outputs a switch signal to the switching component 406 whena time measurement signal has been inputted from the time measurer 412if a select signal has been inputted from the switching component 406.

The switching component 406 switches the method for calculating theon-time. The switching component 406 outputs a select signal to eitherthe first on-time calculator 401, the second on-time calculator 402, orthe switching adjuster 405, whichever has been selected. The switchingcomponent 406 changes the output destination of the select signal when aswitch signal is inputted from the first on-time calculator 401, theswitching adjuster 405, the comparator 407, or the time measurer 412.

When a switch signal is inputted from the first on-time calculator 401in the case where the first on-time calculator 401 is selected, theswitching component 406 changes the selection to the second on-timecalculator 402. When a switch signal is inputted from the comparator 407in the case where the second on-time calculator 402 is selected, theselection is changed to the switching adjuster 405, and when a switchsignal from the time measurer 412 is inputted, the selection is changedto the first on-time calculator 401. Also, when a switch signal isinputted from the switching adjuster 405 in the case where the switchingadjuster 405 is selected, the selection is changed to the first on-timecalculator 401.

FIG. 7 is a diagram illustrating the function of the switching component406. In FIG. 7, the first on-time is indicated by ΔT_(a), and the secondon-time by ΔT_(b). FIG. 7 shows the output pulse waveforms for when thesecond on-time ΔT_(b) was not calculated (output pulse waveform A), whenthe calculated second on-time ΔT_(b) was shorter than half of theinitial period T (output pulse waveform B), and when it was longer thanhalf of the initial period T (output pulse waveform C).

The output pulse waveform A is the output pulse waveform in the casewhere the second on-time ΔT_(b) was not calculated after the calculationof the first on-time ΔT_(a). The selection switching of the switchingcomponent 406 when this output pulse waveform A is generated will bedescribed.

At first, the switching component 406 selects the first on-timecalculator 401. Therefore, when a time measurement signal is inputtedfrom the time measurer 412 at t=t₀, the first on-time calculator 401computes the first on-time ΔT_(a). When the first on-time calculator 401calculates the first on-time ΔT_(a) and a switch signal is outputted tothe switching component 406, the switching component 406 to which theswitch signal was inputted changes the selection to the second on-timecalculator 402. Next, when a time measurement signal is inputted fromthe time measurer 412 at t=t₁, the second on-time calculator 402computes the second on-time ΔT_(b). However, since the off timing isreached without the second on-time ΔT_(b), being calculated, the timemeasurer 412 outputs a switch signal to the switching component 406. Theswitching component 406 to which the switch signal was inputted changesthe selection to the first on-time calculator 401. Next, when a timemeasurement signal is inputted from the time measurer 412 at t=t₂, thefirst on-time calculator 401 computes the first on-time ΔT_(a).

Returning to FIG. 6, the comparator 407 compares the initial period Tset by the initial period setting component 409 with the second on-timeΔT₂ inputted from the second on-time calculator 402. In the case wherethe second on-time ΔT₂ is less than (1/2)·T, the comparator 407 outputsa switch signal to the switching component 406, and outputs the secondon-time ΔT₂ to the off timing setting component 411. In the case wherethe second on-time ΔT₂ is at least (1/2)·T, the second on-time ΔT₂ iscomputed again by the second on-time calculator 402, so no switch signalis outputted.

In FIG. 7, the output pulse waveform B is the output pulse waveform whenthe second on-time ΔT_(b), is calculated after the calculation of thefirst on-time ΔT_(a), and this ΔT_(b) is shorter than (1/2)·T. Until thesecond on-time calculator 402 computes the second on-time ΔT_(b), att=t₁, everything is the same as during the above-mentioned generation ofthe output pulse waveform A. Since the second on-time ΔT_(b) iscalculated, this second on-time ΔT_(b), is inputted to the comparator407. Since the second on-time ΔT_(b), is less than (1/2)·T, thecomparator 407 outputs a switch signal to the switching component 406.The switching component 406 to which the switch signal was inputtedchanges the selection to the switching adjuster 405. The switchingadjuster 405 to which a time measurement signal has been inputted fromthe time measurer 412 at t=t₂ outputs a switch signal to the switchingcomponent 406. The switching component 406 changes the selection to thefirst on-time calculator 401 when a switch signal is inputted from theswitching adjuster 405. Next, when a time measurement signal is inputtedfrom the time measurer 412 at t=t₃, the first on-time calculator 401computes the first on-time ΔT_(a).

The output pulse waveform C is the output pulse waveform when the secondon-time ΔT_(b), has been calculated after the calculation of the firston-time ΔT_(a), this ΔT_(b) is at least (1/2)·T, and the second on-timehas been calculated again, but ΔT_(b), has not be calculated. Until thesecond on-time ΔT_(b), calculated at t=t₁ is inputted to the comparator407, everything is the same as during the above-mentioned generation ofthe output pulse waveform B. Since ΔT_(b), is at least (1/2)·T, thecomparator 407 does not output a switch signal to the switchingcomponent 406. When a time measurement signal is inputted from the timemeasurer 412 at t=t₂, the second on-time calculator 402 computes thesecond on-time ΔT_(b). However, since the off timing is reached withoutthe second on-time ΔT_(b), being calculated, the time measurer 412outputs a switch signal to the switching component 406. The switchingcomponent 406 to which the switch signal was inputted changes theselection to the first on-time calculator 401. Next, when a timemeasurement signal is inputted from the time measurer 412 at t=t₃, thefirst on-time calculator 401 computes the first on-time ΔT_(a).

Returning to FIG. 6, the counter 408 counts the number of times thesecond on-time calculator 402 has calculated the second on-time. Thecount is initialized to n=0 when a reset signal is inputted from thefirst on-time calculator 401, and is incremented by one each time acount signal is inputted from the second on-time calculator 402.

The initial period setting component 409 sets the initial period T,which is the basis period of the PWM signal. The initial period T isdetermined ahead of time on the basis of the user's experience, and inthis embodiment it is set to 0.17 ms.

The on timing setting component 410 sets the time instant of the next ontiming, and the off timing setting component 411 sets the time instantof the next off timing.

The on timing setting component 410 computes the time (1/2)·(T−ΔT₁) fromthe first on-time ΔT₁ inputted from the first on-time calculator 401 andthe initial period T set by the initial period setting component 409.This time is added to the time instant at which the first on-time ΔT₁was computed, and this is set as the time instant of the next on timing.For example, in FIG. 2( a), the calculated time instants t_(a) and t_(c)are set. The set time instants for on timing are inputted to the timemeasurer 412.

The off timing setting component 411 computes the time (1/2)·(T+ΔT₁)from the inputted first on-time ΔT₁ and the initial period T set by theinitial period setting component 409 when the first on-time ΔT₁ has beeninputted from the first on-time calculator 401. This time is added tothe time instant at which the first on-time ΔT₁ was computed, and thisis set as the time instant of the next off timing. The set time instantsfor off timing are inputted to the time measurer 412.

The off timing setting component 411 also calculates the time(1/2)·n·T+ΔT₂ from the inputted second on-time ΔT₂, the initial period Tset by the initial period setting component 409, and the count ninputted from the counter 408 when the second on-time ΔT₂ has beeninputted from the second on-time calculator 402 prior to the set timeinstant of the off timing. This time is added to the time instant atwhich the second on-time ΔT₂ was calculated, and this is set as the timeinstant of the next off timing. For example, in FIG. 2( a), thecalculated time instants t_(b) and t_(d) are set. The set time instantsfor off timing are inputted to the time measurer 412. If the secondon-time ΔT₂ is inputted from the second on-time calculator 402 prior tothe set time instant of the off timing, then the off timing is setagain.

The time measurer 412 keeps track of the time instant of the on timinginputted from the on timing setting component 410 and the time instantof the off timing inputted from the off timing setting component 411.The time measurer 412 outputs a time measurement signal to the pulsesignal generator 413 every time the on timing time instant is checked.The time measurer 412 also outputs a time measurement signal to thepulse signal generator 413 and outputs a switch signal to the switchingcomponent 406 every time the off timing time instant is checked. Also,the time measurer 412 outputs a time measurement signal to the firston-time calculator 401, the second on-time calculator 402, the targetvoltage setting component 404, and the switching adjuster 405 every timethe time (1/2)·T, which is half the initial period T set by the initialperiod setting component 409, has elapsed.

The pulse signal generator 413 generates a pulse signal by switching thelevel to high when an on timing time measurement signal is inputted fromthe time measurer 412, and switching the level is switched to low whenan off timing time measurement signal is inputted from the time measurer412. This pulse signal is outputted as a PWM signal 811 to the switchingelement TR1 of the inverter circuit 3. This pulse signal is alsoinverted and outputted to the switching element TR2 of the invertercircuit 3.

Next, the procedure for generating a PWM signal in the invertercontroller 4 will be described with reference to the flowchart of FIG.8. In the following description, the PWM signal S11 will be used as anexample.

The flowchart in FIG. 8 shows the actual generation processing for a PWMsignal by the inverter controller 4 over time.

First, the first on-time ΔT₁ is calculated with a preset initial periodT being treated as one period (S1). A first pulse waveform in which theon-duration of the calculated first on-time ΔT₁ is located in the middleis generated and outputted (S2).

Next, it is determined whether the time of (1/2)·T has elapsed (S3)after the first on-time ΔT₁ is calculated, or not. If it has not elapsed(S3: No), the flow returns to step S3, but if it has elapsed (S3: Yes),the second on-time ΔT₂ is calculated (S4). Specifically, the secondon-time ΔT₂ is calculated once (1/2)·T has elapsed after the firston-time ΔT₁ is computed.

Next, it is determined whether or not the second on-time ΔT₂ has beencalculated (S5). If it is possible to extend the output pulse waveform,the second on-time ΔT₂ is calculated, but if it is impossible to extendthe output pulse waveform, the second on-time ΔT₂ is not calculated. Ifthe second on-time ΔT₂ has been calculated (S5: Yes), a second pulsewaveform is generated and outputted in which the on-duration of thecalculated second on-time ΔT₂ is located on the leading edge side (stepS6). Specifically, the on-duration of the output pulse waveform isextended.

Next, it is determined whether the second on-time ΔT₂ is at least(1/2)·T (S7) or not. If ΔT₂≧(1/2)·T (S7: Yes), (1/2)·T will elapse fromthe calculation of the second on-time ΔT₂ sooner than the off timing ofthe output pulse waveform, so the flow proceeds to step S3 to determinewhether or not extension of the output pulse waveform again is possible.Specifically, the second on-time ΔT₂ is computed again after the elapseof (1/2)·T since calculation of the second on-time ΔT₂ the first time.This is repeated and the output pulse waveform is extended as long asthe calculated second on-time ΔT₂ is at least (1/2)·T.

The output pulse waveform on the right side in FIG. 2( a) is one inwhich the second on-time ΔT₂ calculated the first time is at least(1/2)·T, and the second on-time ΔT₂ has been calculated again.

In step S7, if ΔT₂<(1/2)·T (S7: No), the output pulse waveform will gooff before (1/2)·T has elapsed since the calculation of the secondon-time ΔT₂, so the flow proceeds to step S8 without the second on-timeΔT₂ being calculated again.

In step S8, it is determined whether a time of T has elapsed after thecalculation of the second on-time ΔT₂ (S8) or not. If it has not elapsedyet (S8: No), the flow returns to step S8, but if it has already elapsed(S8: Yes), the flow returns to step S1. Specifically, the first on-timeΔT₁ is computed to generate the next output pulse waveform after theelapse of T since the calculation of the last second on-time ΔT₂.

The output pulse waveform on the left side in FIG. 2( a) is the one whenthe second on-time ΔT₂ calculated the first time is less than (1/2)·T.The output pulse waveform on the right side in FIG. 2( a) is the onewhen the second on-time ΔT₂ calculated the second time is than (1/2)·T.

In step S5, if the second on-time ΔT₂ has not be calculated (S5: No), itis determined whether the time of (1/2)·T has elapsed after thecalculation of the second on-time ΔT₂ (S9) or not. If it has not elapsedyet (S9: No), the flow returns to step S9, but if it has already elapsed(S9: Yes), the flow returns to step S1. Specifically, the first on-timeΔT₁ is calculated to generate the next output pulse waveform after(1/2)·T has elapsed after the calculation of the second on-time ΔT₂.

As discussed above, with the inverter device 1 pertaining to the presentinvention, a preset initial period T is used as the basic period foreach period of a PWM signal, and the length of each period of the PWMsignal is extended on the basis of an on-duration calculated bycalculation. If the initial period T is set relatively short, then thecalculation of the first on-time ΔT₁ will not end by the time of the ontiming, or calculation cannot be ended and the period cannot be extendedby the off timing. On the other hand, if the initial period T is setrelatively long, there is a possibility that the on-time for maintainingthe output current within the allowable range cannot be obtained.

Therefore, the initial period T is set to a suitable value byexperimentation, simulation, or the like, but a control member forchanging the initial period T may be provided to the inverter device 1so that the user can adjust to the desired value.

Also, with the inverter device 1 pertaining to the present invention,the length of each period of the PWM signal is extended, but since thecontrol period is the same as with the initial period T that is notextended, theoretically there is no decrease in control precision. Also,when period extension is performed, the calculation of the secondon-time ΔT₂ is performed during the on-duration of the output pulsewaveform, so there is substantially no calculation lag.

In a simulation, switching was performed 120 times within one period ofthe output voltage when period extension was not performed at an initialperiod T=0.17 ms, but with the inverter device of this embodiment,switching was performed only 74 times under the same conditions, sothere was a reduction in the number of times switching was necessary. Asimilar effect was obtained with other types of control (feedbackcontrol, two-degree-of-freedom control).

In the first embodiment given above, the first pulse waveform wasgenerated as a pulse waveform in which the on-duration was located inthe middle of one period, but the present invention is not limited tothis. Specifically, the first pulse waveform can be a pulse waveform inwhich the on-duration is located anywhere within one period.

The formulas (1) to (8) given above can be expanded so that the inverterdevice 1 is represented as a linear system in which the input is theon-time ΔT, and in this formula expansion, the first pulse waveform is apulse waveform in which the on-duration is located in the middle of oneperiod, so the range of the integration of the second term in formula(6) is ((1/2)·(T−ΔT), (1/2)·T+ΔT)).

With the present invention, the first pulse waveform can be defined as apulse waveform in which the on-duration is disposed at any locationwithin a period T. Specifically, the first pulse waveform can be definedas a pulse waveform in which the middle of the on-duration is located ath·T (0<h<1), and formula (8) for finding this pulse waveform can befound using the integration range of the second term in formula (6) as((h·T−(1/2)·ΔT), (h·T+(1/2)·ΔT).

Therefore, if a formula corresponding to formula (8) is found in whichthe range of integration of the second term in formula (6) is((1/3)·T−(1/2)·ΔT), ((1/3)·T+(1/2)·ΔT), and a formula is found forfinding ΔT from this formula by the same method as the formula expansionof formulas (9) to (16), then the formula for calculating ΔT will be onein which the first pulse waveform is a pulse waveform in which themiddle of the on-duration is located at (1/3)·T within one period.

Since it is not permitted for the first pulse waveform to be such thatthe on-duration is outside of the period T when the middle of theon-duration of the calculated on-time ΔT is disposed at h·T (0<h<1) inthe period T, at 0<h<1/2, it is necessary to satisfy 0<h·T−(1/2)·ΔT, andat 1/2<h<1, to satisfy h·T+(1/2)·ΔT<T. For example, if the first pulsewaveform is a pulse waveform in which the middle of the on-duration islocated at (1/3)ΔT in one period, then it is necessary to satisfy0<(1/3)·T−(1/2)·ΔT, that is, ΔT<(2/3)·T.

FIG. 9 is a diagram illustrating a case of the first pulse waveform inwhich the on-duration is shifted from the middle of one period(hereinafter this case will be referred to as the “second embodiment”).

As shown in FIG. 9( a), the first pulse waveform is generated as a pulsewaveform in which the middle of the on-duration of the first on-time ΔT₁is located at (1/3)·T. This first pulse waveform comes on after theelapse of the time {(1/3)·T−(1/2)·ΔT₁} since the time instant at whichthe first on-time ΔT₁ was computed (the time instant at the start of oneperiod T), and goes off after the elapse of the time{(1/3)·T−(1/2)·ΔT₁}. Here again, the second on-time ΔT₂ is computed forwhen T is one period after the elapse of the time (1/2)·T.

FIG. 9( b) shows a second pulse waveform having the on-duration of thesecond on-time ΔT₂. This second pulse waveform comes on at the timeinstant at which the second on-time ΔT₂ was calculated (the time instantof (1/2)·T in one period T), and goes off after the elapse of the timeΔT₂. The inverter controller 4 outputs an output pulse waveform in whichthe above two pulse waveforms are combined (see FIG. 9( c)). This outputpulse waveform is a pulse waveform that comes on after the elapse of thetime {(1/3)·T−(1/2)·ΔT₁} since the time instant at which the firston-time ΔT₁ was computed (the time instant at the start of one periodT), and goes off after the elapse of the time {(1/2)·T+ΔT₂}, and inwhich the on-time is {(1/6)·T+(1/2)·ΔT₁+ΔT₂}.

As is clear from FIG. 9( c), the period of this output pulse waveform is(3/2)·T. If the second on-time ΔT₂ is longer than (1/2)·T, that is, ifthe second pulse waveform is at the high level at the end point of theperiod T of the first pulse waveform, then the second on-time ΔT₂ iscomputed again just as in the first embodiment, and the period of theoutput pulse waveform is further extended.

As discussed above, with the present invention the pulse waveform of thePWM signal is generated one pulse at a time by finding the on-time ΔTdisposed at a preset location of a preset period T. In this generationprocessing for one pulse of pulse waveform, the on-time ΔT is recomputedevery time T/2 elapses in each period, and each period is extended inT/2 units on the basis of this recalculation result.

Therefore, the period of the output pulse waveform can be extended againin the second embodiment, just as in the first embodiment. Also, thecalculation period for the first on-time ΔT₁ and the second on-time ΔT₂for extending the period is constant.

However, since not having the on-duration of the first pulse waveformlocated in the middle of one period increases the error in thecalculation formula for the first on-time ΔT₁, the accuracy of thecalculated first on-time is lower in the second embodiment than in thefirst embodiment. Also, with the present invention, where theon-duration of the first pulse waveform is located in the period has tobe preset (that is, the value of h has to be preset), but if thislocation is set near the leading edge side (that is, if h is close to0), there will be problems such as when the starting time instant of theon-duration of the first pulse waveform is exceeded during calculationof the first on-time ΔT₁, or when this on-duration goes beyond oneperiod.

With the present invention, where the on-duration of the first pulsewaveform is disposed in the period T is not directly related to whetheror not the pulse period T can be extended in the PWM signal. Therefore,to avoid such problems as much as possible, it is preferable if thefirst pulse waveform is generated as a pulse waveform in which theon-duration is located in the middle of one period.

Also, in the first and second embodiments, the timing of the calculationof the second on-time ΔT₂ is after the elapse of the time (1/2)ΔT sincethe calculation of the first on-time ΔT₁, but the present invention isnot limited to this timing. The calculation of the second on-time ΔT₂may be performed at a predetermined timing after the calculation of thefirst on-time ΔT₁.

FIG. 10 is a diagram illustrating when the calculation of the secondon-time ΔT₂ is performed at a predetermined timing since the calculationof the first on-time ΔT₁ (hereinafter referred to as the “thirdembodiment”).

FIG. 10( a) shows a first pulse waveform having an on-duration of thefirst on-time ΔT₁ in which the pulse waveform is such that theon-duration is located in the middle of one period. This first pulsewaveform comes on after the elapse of the time (1/2)·(T−ΔT₁) since thetime instant at which the first on-time ΔT₁ was computed (the timeinstant at the start of the period T of the first pulse waveform), andgoes off after the elapse of the time (1/2)·(T+ΔT₁).

In this example, the second on-time ΔT₂ is computed for when T is usedas one period after the elapse of the time (1/3)·T since the timeinstant at the start of the period T of the first pulse waveform. FIG.10( b) shows a second pulse waveform having an on-duration of the secondon-time ΔT₂. This second pulse waveform comes on at the time instant atwhich the second on-time ΔT₂ was computed, and goes off after the elapseof the time ΔT₂. The inverter controller 4 outputs an output pulsewaveform in which the above two pulse waveforms are combined (see FIG.10( c)). This output pulse waveform is a pulse waveform that comes onafter the elapse of the time (1/2)·T−(T−ΔT₁) since the time instant atwhich the first on-time ΔT₁ was computed (the time instant at the startof one period T of the first pulse waveform), and goes off after theelapse of the time {(1/3)·T+ΔT₂}, and in which the on-time is{−(1/6)·T+(1/2)·ΔT₁+ΔT₂}.

As is clear from FIG. 10( c), the period of this output pulse waveformis (4/3)·T (=(1/3)·T+T). If the second on-time ΔT₂ is longer than(2/3)·T, that is, if the second pulse waveform is at the high level atthe end point of the period T of the first pulse waveform, then thesecond on-time ΔT₂ is computed again just as in the first embodiment,and the period of the output pulse waveform is further extended.

If the timing at which the second on-time ΔT₂ is computed is a timeinstant that is beyond the elapse of the time (1/2)·T since the timeinstant at which the first on-time ΔT₁ was computed (the time instant atthe start of the period T of the first pulse waveform), there is aproblem in that the on-duration of the first pulse waveform ends at thetime instant at the start of the on-duration of the second pulsewaveform. To avoid this problem, the timing at which the second on-timeΔT₂ is computed should be close to the time instant after the elapse ofthe time (1/2)·T since the time instant at which the first on-time ΔT₁was computed.

Again in the third embodiment, the period of the output pulse waveformcan be extended. However, since the calculation period for the firston-time ΔT₁ and the second on-time ΔT₂ for extending the period is notconstant, control accuracy is not as good as in the first embodiment.Therefore, the timing at which the second on-time ΔT₂ is computed ispreferably after the elapse of the time (1/2)·T since the calculation ofthe first on-time.

Also, the constitution may be such that the first pulse waveform isgenerated as a pulse waveform in which the on-duration is shifted fromthe middle, and the calculation of the second on-time ΔT₂ is performedat the timing of the middle location of the on-duration of the firstpulse waveform.

FIG. 11 is a diagram illustrating this constitution (hereinafterreferred to as the “fourth embodiment”).

FIG. 11( a) shows a first pulse waveform generated such that thelocation of the on-duration of the first on-time ΔT₁ is shifted from themiddle, and is an example of a pulse waveform in which the middle of theon-duration is located at (1/3)·T. This first pulse waveform comes onafter the elapse of the time {(1/3)·T−(1/2)·ΔT₁} from the time instantat which the first on-time ΔT₁ was computed (the time instant at thestart of one period T), and goes off after the elapse of the time{(1/3)·T+(1/2)·ΔT₁}.

In this example, the second on-time ΔT₂ is computed for when T is oneperiod after the elapse of the time (1/3)·T since the time instant atthe start of the period T of the first pulse waveform. FIG. 11( b) is asecond pulse waveform having an on-duration of the second on-time ΔT₂.This second pulse waveform comes on at the time instant at which thesecond on-time ΔT₂ was calculated, and goes off after the elapse of thetime ΔT₂. The inverter controller 4 outputs an output pulse waveform inwhich the above two pulse waveforms are combined (see FIG. 11( c)). Thisoutput pulse waveform is a pulse waveform that comes on after the elapseof the time {(1/3)·T−(1/2)·ΔT₁} since the time instant at which thefirst on-time ΔT₁ was computed (the time instant at the start of oneperiod T of the first pulse waveform), and goes off after the elapse ofthe time {(1/3)·T+ΔT₂}, and in which the on-time is {(1/2)·ΔT₁+ΔT₂}.

As is clear from FIG. 11( c), the period of this output pulse waveformis (4/3)·T (=(1/3)−T+T). If the second on-time ΔT₂ is longer than(2/3)·T, that is, if the second pulse waveform is at the high level atthe end point of the period T of the first pulse waveform, then thesecond on-time ΔT₂ is computed again just as in the first embodiment,and the period of the output pulse waveform is further extended.

The fourth embodiment is similar to the second embodiment in terms ofavoiding having the location of the on-duration of the first pulsewaveform be near the leading edge side of one period. Also, the periodof the output pulse waveform can be extended in the fourth embodiment aswell, but since this embodiment is the same as the third embodiment inthat the calculation period for the first on-time ΔT₁ and the secondon-time ΔT₂ is not constant, control accuracy is not as good as in thefirst embodiment. Therefore, taking all of this into account, it is alsopreferable in the fourth embodiment if the first pulse waveform isgenerated as a pulse waveform in which the on-duration is located in themiddle of one period.

In the fourth embodiment, rather than performing the calculation of thesecond on-time ΔT₂ at the timing of the middle location of theon-duration of the first pulse waveform, the timing may be at any pointfrom the start of the period of the first pulse waveform.

For the sake of convenience, a single-phase system-linked inverterdevice was described in the above embodiments, but it should go withoutsaying that the present invention can also be applied to the three-phaseinverter device 1′ shown in FIG. 12.

In FIG. 12, circuits that perform the same function as in the inverterdevice 1 in FIG. 1 are numbered the same. The inverter circuit 3 isprovided with a third arm composed of serially connected switchingelements TR5 and TR6 in addition to the first and second arms. Outputlines of U-phase, V-phase, and W-phase output voltage are outputted fromthe respective connection points a, b, and c of the first, second, andthird arms. Inductors L_(F) are serially connected to the three outputlines, and capacitors C_(F) are connected between these output lines.Low-pass filters of the U-phase, V-phase, and W-phase output lines areconstituted by inverted L-shaped connections of the inductors L_(F) andcapacitors C_(F) between the output lines. Therefore, the filter circuit5 has three low-pass filters corresponding to U-phase, V-phase, andW-phase.

Similarly, the output current detector 7 and the system voltage detector8 each comprise three detectors, with U-phase, V-phase, and W-phaseoutput current being detected by the respective detectors, and thesedetection values are inputted to the inverter controller 4.

The inverter controller 4 is equipped with three PWM signal generators41, 42, and 43 corresponding to the first, second, and third arms.Specifically, the inverter controller 4 is equipped with three PWMsignal generators 41, 42, and 43 for generating PWM signals used tocontrol U-phase, V-phase, and W-phase output current. The three PWMsignals outputted from the PWM signal generators 41, 42, and 43 areidentical except that their phases are offset by 120 degrees each.Therefore, the specific function blocks of the PWM signal generators 41,42, and 43 are the same as those shown in FIG. 6, and the invertercontroller 4 will not be described again in detail.

Again with the three-phase inverter device 1′, just as with the inverterdevice 1 in the above embodiments, the inverter controller 4 generatesPWM signals in which the periods of the pulses have been extended.Therefore, the number of times the switching elements TR1 to TR6 of theinverter device 1′ have to be switched is reduced and there is lessswitching loss, so the voltage conversion efficiency can be improved.

In general, as shown in the block diagram of FIG. 13 of the PWM signalgenerators subjected to feedback control, the PWM signal generators ofthis three-phase inverter device 1′ have a dq converter 11, an FBcontroller 12, and an inverse dq converter 13, and comprise a functionof converting triple-phase into dual-phase and generating a controlsignal on the dq axis. With the PWM signal generator shown in FIG. 13,detection values V_(U), V_(V), and V_(W) of the U-phase, V-phase, andW-phase output voltage that have undergone feedback are converted intotwo-phase voltage values v_(d) and v_(q) by the dq converter 11according to the following formula (17), and the FB controller 12 usesthe amount of deviation between these voltage values v_(d) and v_(q) andcontrol target values v_(do) and v_(qo) to generate control signalse_(d) and e_(q). These control signals e_(d) and e_(q) are converted bythe inverse dq converter 13 into three-phase control signals e_(u),e_(v), and e _(w), and a PWM circuit 14 generates from these controlsignals e_(u), e_(v), and e _(w) PWM signals for controlling theU-phase, V-phase, and W-phase output currents.

$\begin{matrix}\left\lbrack {E\; 11} \right\rbrack & \; \\{\begin{bmatrix}V_{d} \\V_{q}\end{bmatrix} = {{\begin{bmatrix}{\cos \; \omega \; t} & {\sin \; \omega \; t} \\{{- \sin}\; \omega \; t} & {\cos \; \omega \; t}\end{bmatrix}\begin{bmatrix}1 & {- \frac{1}{2}} & {- \frac{1}{2}} \\0 & \frac{\sqrt{3}}{2} & {- \frac{\sqrt{3}}{2}}\end{bmatrix}}\begin{bmatrix}V_{u} \\V_{v} \\V_{w}\end{bmatrix}}} & (17)\end{matrix}$

With the three-phase inverter device 1′ pertaining to the presentinvention, since calculation processing for generating PWM signals canalso be performed on the dq axis, the block diagram corresponding toFIG. 13 becomes FIG. 14. In FIG. 14, an on-time calculation circuit 15and a pulse waveform generation circuit 16 correspond to the overallfunction block diagram of the inverter controller 4 in FIG. 6, and thecalculated two-phase on-times ΔT_(d) and ΔT_(q) are converted by theinverse dq converter 13 into three-phase on-times ΔT_(u), ΔT_(v), andΔT_(w).

Again with the three-phase inverter device 1′ pertaining to the presentinvention, the control target values for output current can be inputtedas control target values on the dq axis, so the concept of dq conversioncan be applied to the three-phase inverter device pertaining to thepresent invention just as with a conventional three-phase inverterdevice. Furthermore, since the modeling error that occurs in makingoutput voltage discrete is suppressed by converting the basic wavecomponent of the control signal into a DC component by using dqconversion, less error can be achieved with systems having a longswitching period.

With the embodiments above, the description was of a system-linkedinverter device in which the system was the load, but the presentinvention can also be applied to an inverter device for supplying ACpower to a load other than a system, such as an inverter device used formotor drive. However, the present invention functions more effectivelywhen the need for higher efficiency is given more emphasis than higheraccuracy or faster response.

Also, the PWM signal generator of the present invention is not limitedto an inverter device, and can also be applied to a system where theeffect is to lengthen as much as possible the period of inputted PWMsignals under set conditions.

Also, with the embodiments above, the description was of a case of usingthe PWM signal generator of the present invention in a system-linkedinverter device, but the PWM signal generator of the present inventionmay instead be realized by having a computer read a program, forgenerating PWM signals by the above-mentioned method in a conventionalPWM signal generator, from a ROM or other such recording medium to whichsaid program has been recorded in computer-readable fashion, and thenexecuting that program.

1. A PWM signal generator, comprising: a first pulse waveform generator for generating a first pulse waveform corresponding to one period of a first pulse signal; a second pulse waveform generator for generating a second pulse waveform when a preset delay time elapses after a start of generation of the first pulse waveform, the second pulse waveform corresponding to one period of a second pulse signal; and a PWM signal generator for generating a PWM signal based on a composite pulse waveform in which a whole of the first pulse waveform generated by the first pulse waveform generator is combined with a whole of the second pulse waveform generated by the second pulse waveform generator; wherein the first pulse waveform generator generates a next first pulse waveform at an end of the composite pulse waveform.
 2. The PWM signal generator according to claim 1, wherein the first pulse waveform becomes a high level in a middle portion of a first pulse period corresponding to the period of the first pulse signal and becomes a low level at both ends of the first pulse period, wherein the second pulse waveform becomes a high level in a former part of a second pulse period corresponding to the period of the second pulse signal and becomes a low level in a latter part of the second pulse period, and wherein the composite pulse waveform is a same type of waveform as the first pulse waveform constructed by connecting the second pulse waveform to a high level duration of the first pulse waveform.
 3. The PWM signal generator according to claim 2, wherein the first pulse period is equal to the second pulse period.
 4. The PWM signal generator according to claim 2, wherein the high level duration of the first pulse waveform is disposed in a middle of the first pulse period.
 5. The PWM signal generator according to claim 4, wherein the delay time satisfies a condition that the generation of the second pulse waveform starts in a duration in which the first pulse waveform generated by the first pulse waveform generator is a high level.
 6. The PWM signal generator according to claim 5, wherein the delay time is one-half of the first pulse period.
 7. The PWM signal generator according to claim 2, wherein the first pulse waveform generator includes: a first on-time calculator for computing, at a start of the first pulse period, a first on-time in which the first pulse waveform is to be a high level; and a first inversion timing decider for determining a first inversion timing at which a level of the first pulse waveform inverts from a low level to a high level in the first pulse period, based on the first on-time and a position of the high level in the first pulse period; wherein the second pulse waveform generator includes: a second on-time calculator for computing, after a elapse of the delay time after a start of the first pulse period, a second on-time in which the second pulse waveform is to be at a high level; and a second inversion timing decider for determining, based on the second on-time, a second inversion timing at which a level of the second pulse waveform inverts from a high level to a low level in the second pulse period in which the second on-time has been computed, and wherein the PWM signal generation means includes: an inversion timing detector for detecting the first and second inversion timings with reference to a start timing of the first pulse period; and a PWM signal output unit for setting an output level to the low level at the start of the first pulse period, subsequently inverting the output level to the high level when the first inversion timing is detected, inverting thereafter the output level to the low level when the second inversion timing is detected, thereby generating a pulse signal in which the first pulse waveform and the second pulse waveform are combined, for outputting this pulse signal as the various pulses of the PWM signal.
 8. The PWM signal generator according to claim 7, wherein the first inversion timing decider determines, as the first inversion timing, a point when a remaining time, obtained by subtracting one-half of the computed first on-time from a time at the middle position of the high level in the first pulse period, has elapsed from a start of calculation of the first on-time, every time the first on-time is computed, and wherein the second inversion timing determination means determines, as the second inversion timing, a point when the computed second on-time has elapsed from a start of calculation of the second on-time, every time the second on-time is computed.
 9. The PWM signal generator according to claim 2, further comprising: a determiner for determining whether the level of the second pulse waveform is a high level or not, every time a period of the first pulse waveform ends; and a pulse waveform regenerator for causing, only when the level of the second pulse waveform at an end of a period of the first pulse waveform is a high level, the second pulse waveform generator to generate a second pulse waveform again at an end of a period of the first pulse waveform, wherein the PWM signal generator generates a PWM signal based on a composite pulse waveform in which the first pulse wave form is combined with the generated second pulse waveform and the regenerated second pulse waveform.
 10. The PWM signal generator according to claim 9, further comprising: a second determiner for determining whether a level of the second pulse waveform regenerated at an end of a period of the previously generated second pulse waveform is a high level or not when the generation of the second pulse waveform is performed again by the pulse waveform regenerator, wherein the pulse waveform regenerator repeats an operation of causing the second pulse waveform generator to generate a second pulse waveform again at an end of a period of the previously generated second pulse waveform, until a level of the second pulse waveform regenerated at an end of a period of the previously generated second pulse waveform reaches a low level, and wherein the PWM signal generator generates a PWM signal based on a composite pulse waveform in which the first pulse waveform is combined with the generated second pulse waveform and one or more regenerated second pulse waveforms.
 11. The PWM signal generator according to claim 10, wherein the first pulse waveform generator includes: a first-on time calculator for computing, at a start of the first pulse period, a first on-time in which the first pulse waveform is to be at a high level; and a first inversion timing decider for determining a first inversion timing at which a level of the first pulse waveform inverts from a low level to a high level in the first pulse period based on the first on-time and a position of a high level in the first pulse period, wherein the second pulse waveform generator includes: a second on-time calculator for computing a second on-time in which the second pulse waveform is to be at a high level, after a elapse of the delay time from a start of the first pulse period, and if a generation of the second pulse waveform is performed again by the pulse waveform regenerator, at an end of the first pulse period and at an end of the period of the previously generated second pulse waveform; and a second inversion timing decider for determining, based on the second on-time that has been last computed by the second on-time calculator, a second inversion timing at which a level of the second pulse waveform inverts from a high level to a low level in the second pulse period in which the second on-time has been computed, and wherein the PWM signal generator includes: an inversion timing detector for detecting the first and second inversion timings with reference to a start time of the first pulse period; and a PWM signal output unit for setting an output level to a low level at a start of the first pulse period, subsequently inverting the output level to the high level when the first inversion timing is detected, holding the output level at the high level on the basis of the one or more generated second pulse waveforms, subsequently inverting the output level to the low level when the second inversion timing is detected, thereby generating a pulse signal in which the first pulse waveform and the one or more second pulse waveform are combined, for outputting this pulse signal as the various pulses of the PWM signal.
 12. The PWM signal generator according to claim 11, wherein the first inversion timing decider determines, as the first inversion timing, a point when the remaining time, obtained by subtracting one-half of the computed first on-time from the time at the middle position of the high level in the first pulse period, has elapsed from the start of calculation of the first on-time, every time the first on-time is computed, and wherein the second inversion timing decider determines, as the second inversion timing, a point when the second on-time last computed has elapsed from the start of the last calculation of the second on-time.
 13. The PWM signal generator according to claim 7, wherein the first on-time calculator computes the first on-time by using a first calculation formula for finding a solution to a first state equation in which an input variable is the first on-time of the first pulse waveform and which is derived from a state equation in which a state variable inputted to a control object is the first pulse waveform, and wherein the second on-time calculator computes the second on-time by using a second calculation formula for finding a solution to a second state equation in which an input variable is the second on-time of the second pulse waveform and which is derived from a state equation in which the state variable inputted to the control object is the second pulse waveform.
 14. An inverter device, comprising: a DC power supply that outputs DC voltage; a bridge circuit which inversely converts the DC voltage outputted from the DC power supply into AC voltage, and in which a plurality of switching elements are bridge-connected; a control circuit that controls the inverse conversion operation of the bridge circuit by controlling an on/off operation of the plurality of switching elements; a filter circuit that removes switching noise included in the AC voltage outputted from the bridge circuit; and a transformer that receives the AC voltage outputted from the filter circuit for applying a transformed voltage to a load, wherein the control circuit includes the PWM signal generator according to claim 1, and controls an on/off operation of the plurality of switching elements by means of PWM signals generated by the PWM signal generator.
 15. The inverter device according to claim 14, wherein the DC power supply comprises a solar cell, the bridge circuit comprises a three-phase bridge circuit, and the AC voltage outputted from the transformer is three-phase AC voltage outputted in connection with a commercial power system. 